Method for determining a map, device manufacturing method, and lithographic apparatus

ABSTRACT

A method according to one embodiment of the invention includes determining a map of a second part of a substrate belonging to a group of substrates. The method includes measuring a first part of at least one substrate belonging to the group to create an average profile map or average height map and computing a map of the second part of the substrate belonging to the group, based on the average profile map or the average height map. The computed map is stored for use during a later determination of a height or tilt of a substrate from the group.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/013,202 filed Dec. 16, 2004, which is a continuation-in-part of andclaims benefit of U.S. patent application Ser. No. 10/736,987 filed Dec.17, 2003, each application of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to lithographic projection apparatus andmethods.

BACKGROUND

The term “patterning structure” as here employed should be broadlyinterpreted as referring to any structure or field that may be used toendow an incoming radiation beam with a patterned cross-section,corresponding to a pattern that is to be created in a target portion ofa substrate; the term “light valve” can also be used in this context. Itshould be appreciated that the pattern “displayed” on the patterningstructure may differ substantially from the pattern eventuallytransferred to e.g. a substrate or layer thereof (e.g. where pre-biasingof features, optical proximity correction features, phase and/orpolarization variation techniques, and/or multiple exposure techniquesare used). Generally, such a pattern will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit or other device (see below). A patterningstructure may be reflective and/or transmissive. Examples of patterningstructure include:

A mask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

A programmable mirror array. One example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, theundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. An array of grating light valves (GLVs) may also be used in acorresponding manner, where each GLV may include a plurality ofreflective ribbons that can be deformed relative to one another (e.g. byapplication of an electric potential) to form a grating that reflectsincident light as diffracted light. A further alternative embodiment ofa programmable mirror array employs a matrix arrangement of very small(possibly microscopic) mirrors, each of which can be individually tiltedabout an axis by applying a suitable localized electric field, or byemploying piezoelectric actuation means. For example, the mirrors may bematrix-addressable, such that addressed mirrors will reflect an incomingradiation beam in a different direction to unaddressed mirrors; in thismanner, the reflected beam is patterned according to the addressingpattern of the matrix-addressable mirrors. The required matrixaddressing can be performed using suitable electronic means. In both ofthe situations described hereabove, the patterning structure cancomprise one or more programmable mirror arrays. More information onmirror arrays as here referred to can be gleaned, for example, from U.S.Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193 and PCT patentapplications WO 98/38597 and WO 98/33096, which documents areincorporated herein by reference. In the case of a programmable mirrorarray, the support structure may be embodied as a frame or table, forexample, which may be fixed or movable as required.

A programmable LCD panel. An example of such a construction is given inU.S. Pat. No. 5,229,872, which is incorporated herein by reference. Asabove, the support structure in this case may be embodied as a frame ortable, for example., which may be fixed or movable as required.

The support structure supports (i.e., bears the weight of) thepatterning structure. It holds the patterning structure in a waydepending on the orientation of the patterning structure, the design ofthe lithographic apparatus, and other conditions (such as, for example,whether or not the patterning structure is held in a vacuumenvironment). The support can be using mechanical clamping, vacuum, orother clamping techniques (for example, electrostatic clamping, possiblyunder vacuum conditions). The support structure may be a frame or atable, for example, which may be fixed or movable as required and whichmay ensure that the patterning structure is at a desired position, forexample with respect to the projection system. Any use of the terms“reticle” or “mask” herein may be considered synonymous with the moregeneral term “patterning structure”. For purposes of simplicity, therest of this text may, at certain locations, specifically direct itselfto examples involving a mask (or “reticle”) and mask table (or “reticletable”); however, the general principles discussed in such instancesshould be seen in the broader context of the patterning structure ashereabove set forth.

A lithographic apparatus may be used to apply a desired pattern onto asurface (e.g. a target portion of a substrate). Lithographic projectionapparatus can be used, for example, in the manufacture of integratedcircuits (ICs). In such a case, the patterning structure may generate acircuit pattern corresponding to an individual layer of the IC, and thispattern can be imaged onto a target portion (e.g. comprising one or moredies and/or portion(s) thereof) on a substrate (e.g. a wafer of siliconor other semiconductor material) that has been coated with a layer ofradiation-sensitive material (e.g. resist). In general, a single waferwill contain a whole matrix or network of adjacent target portions thatare successively irradiated via the projection system (e.g. one at atime).

Among current apparatus that employ patterning by a mask on a masktable, a distinction can be made between two different types of machine.In one type of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionat once; such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus—commonly referred to as a step-and-scanapparatus—each target portion is irradiated by progressively scanningthe mask pattern under the projection beam in a given referencedirection (the “scanning” direction) while synchronously scanning thesubstrate table parallel or anti-parallel to this direction; since, ingeneral, the projection system will have a magnification factor M(generally <1), the speed V at which the substrate table is scanned willbe a factor M times that at which the mask table is scanned. Aprojection beam in a scanning type of apparatus may have the form of aslit with a slit width in the scanning direction. More information withregard to lithographic devices as here described can be gleaned, forexample, from U.S. Pat. No. 6,046,792, which is incorporated herein byreference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (e.g.resist). Prior to this imaging procedure, the substrate may undergovarious other procedures such as priming, resist coating, and/or a softbake. After exposure, the substrate may be subjected to other proceduressuch as a post-exposure bake (PEB), development, a hard bake, and/ormeasurement/inspection of the imaged features. This set of proceduresmay be used as a basis to pattern an individual layer of a device (e.g.an IC). For example, these transfer procedures may result in a patternedlayer of resist on the substrate. One or more pattern processes mayfollow, such as deposition, etching, ion-implantation (doping),metallization, oxidation, chemo-mechanical polishing, etc., all of whichmay be intended to create, modify, or finish an individual layer. Ifseveral layers are required, then the whole procedure, or a variantthereof, may be repeated for each new layer. Eventually, an array ofdevices will be present on the substrate (wafer). These devices are thenseparated from one another by a technique such as dicing or sawing,whence the individual devices can be mounted on a carrier, connected topins, etc. Further information regarding such processes can be obtained,for example, from the book “Microchip Fabrication: A Practical Guide toSemiconductor Processing”, Third Edition, by Peter van Zant, McGraw HillPublishing Co., 1997, ISBN 0-07-067250-4.

A substrate as referred to herein may be processed before or afterexposure: for example, in a track (a tool that typically applies a layerof resist to a substrate and develops the exposed resist) or a metrologyor inspection tool. Where applicable, the disclosure herein may beapplied to such and other substrate processing tools. Further, thesubstrate may be processed more than once (for example, in order tocreate a multi-layer IC), so that the term substrate as used herein mayalso refer to a substrate that already contains multiple processedlayers.

The term “projection system” should be broadly interpreted asencompassing various types of projection system, including refractiveoptics, reflective optics, and catadioptric systems, for example. Aparticular projection system may be selected based on factors such as atype of exposure radiation used, any immersion fluid(s) or gas-filledareas in the exposure path, whether a vacuum is used in all or part ofthe exposure path, etc. For the sake of simplicity, the projectionsystem may hereinafter be referred to as the “lens.” The radiationsystem may also include components operating according to any of thesedesign types for directing, shaping, reducing, enlarging, patterning,and/or otherwise controlling the projection beam of radiation, and suchcomponents may also be referred to below, collectively or singularly, asa “lens.”

Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCTApplication No. WO 98/40791, which documents are incorporated herein byreference.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index (e.g.water) so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. The use of immersiontechniques to increase the effective numerical aperture of projectionsystems is well known in the art.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange 5-20 nm), as well as particle beams (such as ion or electronbeams).

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beexplicitly understood that such an apparatus has many other possibleapplications. For example, it may be employed in the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, liquid-crystal display panels, thin-film magneticheads, DNA analysis devices, etc. The skilled artisan will appreciatethat, in the context of such alternative applications, any use of theterms “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “substrate” and “target portion”,respectively.

A substrate may not be perfectly flat, and leveling the substrate aswell as possible in the focal plane of the projection system may bedifficult, especially for target portions at or close to a periphery ofthe substrate.

SUMMARY

A method of determining a map of a surface according to an embodiment ofthe invention includes measuring a first part of a substrate belongingto a group of substrates and, based on a result of the measuring,computing a map of a second part of at least one substrate belonging tothe group, where the first part at least partially overlaps with thesecond part. Methods according to other embodiments also includedetermining a height or tilt of a substrate belonging to the group andapplying the map to correct the determined height or tilt. Apparatus(e.g. measurement and/or lithographic apparatus) and computer-readablemedia including instructions for executing such a method are alsodisclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in which:

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention;

FIG. 2 a schematically depict a part of a lithographic projectionapparatus according to an embodiment of the invention;

FIG. 2 b schematically depicts a substrate divided in a number of targetportions;

FIG. 3 schematically depicts a substrate positioned on a chuck accordingto an embodiment of the invention;

FIG. 4 schematically depicts a sensor according to an embodiment of theinvention;

FIG. 5 a and 5 b schematically depict a top view of a substrateaccording to an embodiment of the invention; and

FIG. 6 schematically depicts two average height maps.

In the Figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION

Embodiments of the invention include, for example, methods and apparatusthat may be used to enable a more accurate determination of the heightor tilt of parts of a substrate that cannot be measured by a levelsensor.

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

A radiation system configured to supply (e.g. having structure capableof supplying) a projection beam of radiation (e.g. UV or EUV radiation).In this particular example, the radiation system RS comprises aradiation source SO, a beam delivery system BD, and an illuminationsystem IL including adjusting structure AM for setting an illuminationnode, an integrator IN, and condensing optics CO;

A support structure configured to support a patterning structure capableof patterning the projection beam. In this example, a first object table(mask table) MT is provided with a mask holder for holding a mask MA(e.g. a reticle), and is connected to a first positioning structure PMfor accurately positioning the mask with respect to item PL;

A second object table (substrate table) configured to hold a substrate.In this example, substrate table WT is provided with a substrate holderfor holding a substrate W (e.g. a resist-coated semiconductor wafer),and is connected to a second positioning structure PM for accuratelypositioning the substrate with respect to item PL and (e.g.interferometric) measurement structure IF, which is configured toaccurately indicate the position of the substrate and/or substrate tablewith respect to lens PL; and

A projection system (“lens”) configured to project the patterned beam.In this example, projection system PL (e.g. a refractive lens group, acatadioptric or catoptric system, and/or a mirror system) is configuredto image an irradiated portion of the mask MA onto a target portion C(e.g. comprising one or more dies and/or portion(s) thereof) of thesubstrate W. Alternatively, the projection system may project images ofsecondary sources for which the elements of a programmable patterningstructure may act as shutters. The projection system may also include amicrolens array (MLA), e.g. to form the secondary sources and to projectmicrospots onto the substrate.

As here depicted, the apparatus is of a transmissive type (e.g. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (e.g. with a reflective mask). Alternatively, theapparatus may employ another kind of patterning structure, such as aprogrammable mirror array of a type as referred to above.

The source SO (e.g. a mercury lamp, an excimer laser, an electron gun, alaser-produced plasma source or discharge plasma source, or an undulatorprovided around the path of an electron beam in a storage ring orsynchrotron) produces a beam of radiation. This beam is fed into anillumination system (illuminator) IL, either directly or after havingtraversed a conditioning structure or field. For example, a beamdelivery system BD may include suitable directing mirrors and/or a beamexpander. The illuminator IL may comprise an adjusting structure orfield AM for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in the beam, which may affect the angular distribution ofthe radiation energy delivered by the projection beam at, for example,the substrate. In addition, the apparatus will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the beam PB impinging on the mask MA has a desireduniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source SO may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source SO is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable direction mirrors); this latter scenario is oftenthe case when the source SO is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed (alternatively, having been selectivelyreflected by) the mask MA, the beam PB passes through the lens PL, whichfocuses the beam PB onto a target portion C of the substrate W. With theaid of the second positioning structure PW (and interferometricmeasuring structure IF), the substrate table WT can be moved accurately,e.g. so as to position different target portions C in the path of thebeam PB. Similarly, the first positioning structure PM (and possiblyanother position sensor) can be used to accurately position the mask MAwith respect to the path of the beam PB, e.g. after mechanical retrievalof the mask MA from a mask library, or during a scan. In general,movement of the object tables MT, WT will be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. However, inthe case of a wafer stepper (as opposed to a step-and-scan apparatus)the mask table MT may just be connected to a short stroke actuator, ormay be fixed. Mask MA and substrate W may be aligned using maskalignment marks M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in several different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected at once (i.e. in a single “flash”)onto a target portion C. The substrate table WT is then shifted in the xand/or y directions so that a different target portion C can beirradiated by the beam PB. In step mode, the maximum size of theexposure field may limit the size of the target portion C imaged in asingle static exposure;

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so-called “scandirection”, e.g. the y direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image. Concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the lens PL (typically,M=¼ or ⅕). The velocity and direction of the substrate table WT relativeto the mask table MT may be determined by the magnification,demagnification, and/or image reversal characteristics of the projectionsystem PL. In this manner, a relatively large target portion C can beexposed, without having to compromise on resolution. In scan mode, themaximum size of the exposure field may limit the width (in thenon-scanning direction) of the target portion in a single dynamicexposure, whereas the length of the scanning motion may determine theheight (in the scanning direction) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning structure, and the substrate table WTis moved or scanned while a pattern imparted to the projection beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning structureis updated as required after each movement of the substrate table WT orin between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning structure, such as a programmable mirror arrayof a type as referred to above.

Combinations and/or variations on the above-described modes of use orentirely different modes of use may also be employed.

Discussion of the prior art will be done with reference to FIG. 2 b,that schematically depicts a substrate W, divided in a number of targetportions C_(i) (i=1, 2, . . . ). The term substrate W used herein shouldbe broadly interpreted as the usable area of the substrate W forexposures, e.g. excluding the focus edge clearance region.

Imaging a pattern onto a substrate W is usually done with opticalelements, such as lenses or mirrors. In order to generate a sharp image,a layer of resist on the substrate should be in the focal plane of theoptical elements. Therefore, according to the prior art, the height ofthe target portion C_(i) that is to be exposed is measured. Based onthese measurements, the height of the substrate W with respect to theoptical elements is adjusted, e.g. by moving the substrate table onwhich the substrate W is positioned (not shown). Since a substrate W isnot a perfectly flat object, it may not be possible to position thelayer of resist exactly in the focal plane of the optics for the wholetarget portion C_(i), so the substrate W may only be positioned as wellas possible.

In order to position the top surface of the substrate W in the focalplane as well as possible, the orientation of the substrate W may alsobe altered. The substrate table may be translated, rotated or tilted, inall six degrees of freedom, in order to position the layer of resist inthe focal plane as well as possible. This may be necessary for alltarget portions C_(i), but especially for the target portions situatedon the edge of the substrate, since these target portions are usuallyslanting, due to the presence of the edge.

In order to determine the best positioning of the substrate W withrespect to the optical elements, the surface of the substrate W may bemeasured using a level sensor, as for instance described in U.S. Pat.No. 5,191,200. This procedure may be done during exposure (on-the-fly),by measuring the part of the substrate W that is being exposed or isnext to be exposed, but the surface of the substrate W may also bemeasured in advance. This latter approach can also be done at a remoteposition. In the latter case, the results of the measurements may bestored in the form of a so-called height map and used during exposure toposition the substrate W with respect to the focal plane of the opticalelements.

In both cases, the top surface of the substrate W may be measured with alevel sensor that determines the height of a certain area. This area maybe a slit 10, having a width about equal to the width of the targetportion C_(i) and having a length that is only part of the length oftarget portion C_(i), as is shown in FIG. 2. The height map of a targetportion C_(i) may be measured by scanning the target portion C_(i) inthe direction of the arrow A. The level sensor determines the height ofthe substrate by applying a multi-spot measurement, such as for instancea 4- or a 8-spot measurement. The spots 11 are spread over the slit 10and, based on the measurements obtained from the different spots 11, aheight map is constructed.

However, it may be difficult or impossible to apply successfully aprocedure as described above if the target portion C_(i) is on the edgeof the substrate W and the area in which the level sensor projects itsspots is partially outside the substrate W, as is the case in the targetportion C₁ that is measured in FIG. 2. If the slit 10 is moved in thedirection of the arrow A, a number of spots 11 are partially or totallyprojected outside the surface of the substrate W, and correct spotmeasurements may not be possible for that part of the target portion C₁.The substrate height determination for target portion C₁ can then bequalitatively poor due to improper coverage of the target portion C₁with spot measurements, or may even fail if a required combination ofspot measurements is not available. Especially the determination of thetilt of the target portion C₁ may fail or be qualitatively bad when thecombination of spots 11 of the level sensor projected onto the substrateW is less than required. The same problem may occur with target portionC₅.

Since a target portion C may comprise several chips, correct projectionof the pattern is desired. Even if part of the target portion C is noton the substrate W, some chips may be completely on the substrate andcould result in useful products.

According to the prior art, the height of the target portions C, or partof the target portion C, that can no longer be correctly measured usingthe level sensor was determined based on extrapolation of the height mapconstructed in neighboring areas on the substrate W. As can be seen inFIG. 2, it may not be possible to measure correctly the top part of thetarget portion C₁ being scanned, since a required combination of spotswill not be completely within the boundaries of the substrate W.Therefore, the height map of that part of the target portion C₁ wasconstructed based on previous measurements done in surrounding areas,such as in the neighboring target portion C₂, or done at the lower partof the same target portion C₁. The results of the height map of thoseneighboring areas were constructed using extrapolation. The tilt in thearea that could not be measured was assumed to equal the tilt determinedin a neighboring area, while the height was extrapolated using linearextrapolation.

However, this extrapolation is only correct if the neighboring areas onthe substrate W are on a (substantially) flat plane. It is observed thatthis condition is not always the case, and in particular at the edge ofthe substrate W the surface is often curved. This curvature may becaused by the shape of the substrate W itself, but may also be caused bythe treatment the substrate W has been subjected to, such as polishingsteps, or may be caused by the underlying substrate stage to which thesubstrate W is clamped. The curvature could have different causes, sothe curvature could subsequently have different shapes.

In FIG. 2 a, a section 14 between the mask MA and the substrate table WTof the lithographic projection apparatus is shown. In the section 14,the so-called projection system PL (e.g. as shown in FIG. 1) is present.The projection system PL contains several elements to guide andcondition the projection beam PB of radiation, as is known to personsskilled in the art. After passing the projection system PL, theprojection beam PB of radiation hits the surface of the substrate W onthe substrate table WT. The wafer table WT will be discussed below inmore detail with reference to FIG. 3.

The substrate table WT is connected to actuators 23. These actuators 23are connected to a control device 6 with a central processing unit (CPU)8 and a memory 9. The central processing unit 8 further receivesinformation from sensors 25 measuring the actual position of the wafertable WT or wafer table holder, e.g. electrically (capacitive,inductive) or optically (e.g. interferometrically, as shown in FIG. 1).

The CPU 8 also receives input from a sensor 15 which measures the heightand/or tilt information from the target area on the wafer where theprojection beam PB hits the substrate surface. This sensor 15 willhereinafter further referred to as level sensor, LS. The level sensormay be, for example, an optical sensor; alternatively, a pneumatic orcapacitive sensor (for example) is conceivable. It may be desirable forthe sensor to be an optical sensor making use of Moire patterns formedbetween the image of a projection grating reflected by the wafer surfaceand a fixed detection grating, as described in U.S. Pat. No. 5,191,200.It may be desirable for the level sensor 15 to measure the verticalheight of a plurality of positions simultaneously and/or to measure theaverage height of a small area for each position, so averagingnon-flatness of high spatial frequencies. This arrangement comprises alight source 2, projection optics (not shown), and detection optics (notshown). The sensor 15 generates a height dependent signal which is fedto the CPU 8.

A level sensing method may use a multi-spot sensor 15 (for instance, 4or 8 sensing areas) and measure the average height of a small area, suchas a slit 10, as is shown in FIG. 2 b, and will be further discussedbelow.

As already discussed above, FIG. 2 b shows a part of a substrate Wdivided in different target portions C_(i) (i=1, 2, . . . ). Since thesubstrate W has a rounded shape and the target portions C are formed asrectangles, the target portions C situated near the edge are not allcompletely on the substrate W. When a height map of the substrate W isdetermined by a level sensor (for instance, by scanning a target portionC with a measurement area, such as a slit 10), the height and/or tiltcannot be determined successfully for the whole target portion C₁. Whenthe slit 10 is moved in the direction of the arrow A, a number of spots11 are partially or totally projected outside the surface of thesubstrate W, and correct spot measurements may not be possible for thatpart of the target portion C₁. The substrate height determination fortarget portion C₁ can then be qualitatively poor, due to impropercoverage of the target portion C₁ with spot measurements, or may evenfail if the combination of spot measurements available is less thanrequired. Especially the determination of the tilt of the target portionC, may fail or be qualitatively bad when a combination of spots 11 ofthe level sensor projected onto the substrate W is less than required.

According to the prior art, the height map of parts of the substrate Wthat cannot be measured accurately with the level sensor may beconstructed by extrapolation of the height map of neighboring areas thatcould still be measured accurately. These neighboring areas may beneighboring target portions C₂ or C₃, but may also be parts of the sametarget portion C₁ that could still be measured accurately.

Since a substrate W usually exhibits a curvature near the edge of thesubstrate W, it is possible that the height map will not be determinedaccurately by sheer extrapolation of the height map determined foradjacent areas.

Some embodiments of the invention relate to the understanding thatsubstrates W that have been subjected to similar circumstances (such assubstrates W from a certain batch, substrates W that have been subjectedto similar processing steps, and/or substrates W that are clamped to asame chuck) may be expected to have similar curvatures near the edge ofthe substrate W. Once the shape of this curvature is known,extrapolation can be done more accurately. The shape of the substrate isdescribed by means of an average height map. This average height mapcontains information about the general shape of a certain substrateunder certain situations. The extrapolation from neighboring areas canbe done more accurately by using the information from such an averageheight map.

An average height map may be constructed and may be used to determine aheight map for areas on the substrate W that cannot be measuredaccurately by the level sensor. The average height map could be based ondifferent information, such as the known shape of substrates W from abatch. Also, known effects of treatment steps supplied to the substrateW can be the basis for constructing an average height map, such as amethod used to polish the substrate W. The average height map couldfurther be based on a flatness map of the chuck on which the substrate Wis positioned. Even the curvature of the underlying stone could be takeninto account.

FIG. 3 shows a substrate W positioned on a chuck 20, comprising asubstrate table WT, a mirror block MB, an air cushion generating device21, and a stone 22. The mirror block MB is provided with mirrors thatmay be used by interferometers (not shown in FIG. 3) to determine theposition of the chuck, and indirectly the substrate W. The stone 22 isassumed to be a steady base on which an air cushion generating device 21is positioned. The mirror block MB may be positioned on top of the aircushion generating device 21, e.g. in order to position the mirror blockMB free of vibrations. Actuators 23 are positioned in between the mirrorblock MB and the air cushion generating device 21 to accurately positionthe mirror block MB. On the mirror block MB, a substrate table WT ispositioned, provided with a plurality of pimples on which substrate Wcan be positioned.

FIG. 3 schematically shows that the top surface of the substrate W maynot be completely flat. In fact, also the lower surface of the substrateW may not be completely flat. However, since only the resulting shape ofthe top surface is relevant here, only the top surface here is depictedas a non-flat surface. The substrate W is clamped to the substrate tableWT by means of a clamping mechanism (not shown). Such a clampingmechanism may for instance be an electromagnetic clamping mechanism or avacuum clamping mechanism.

As already mentioned, the curvature of the substrate W near the edge ofthe substrate W could have a number of causes, as for instance, theshape of the substrate W itself, the shape of the underlying structure(such as the substrate table WT), or the effect of a clamping mechanismused to clamp the substrate to the chuck 20. According to one embodimentof the invention, all these causes are taken into accountsimultaneously. However, the different causes could also be dealt withseparately as will be explained below.

FIG. 3 further shows a processing unit 31 that is arranged tocommunicate with a sensor 30 and a memory unit 32. The sensor 30 isarranged to measure the surface of the substrate W in a more accurateway than the level sensor that is discussed above, i.e. the sensor 30 iscapable of measuring the height of the substrate W also in the vicinityof the edge of the substrate W. Sensor 30 could for instance be asingle-spot level sensor, as is depicted in FIG. 4, but could also be anair gauge or an external profilometer.

FIG. 4 depicts a beam generator 40 that projects a light beam towardsthe surface of the substrate W. The beam is reflected by the surface ofthe substrate W, and the reflected light beam is detected by aphoto-detector 41. The substrate W is moved laterally with respect tothe beam generator 40 and the photo-detector 41. The reflected beamimpinging on the photo-detector 41 provides information about the heightof the substrate W at the position where the light beam is reflected, aswill readily be understood by a person skilled in the art. By thelateral movement of the substrate W, thus, a height map can be derived,e.g. a set of data comprising height information as a function ofposition on substrate W. Other sensors 30 may be used instead, such asinterferometric sensors, that in FIG. 3 are all represented by sensor30.

It may be desirable for sensor 30 to measure the height map of thesubstrate near the edge of the substrate. The measurements done bysensor 30 are processed by the processing unit 31. Based on themeasurements done by sensor 30, the processing unit 31 constructs anaverage height map that is stored in the memory unit 32.

The average height map may contain information about the shape of acertain type of substrate W. As will be discussed below, the informationmay be based on measurements on one or more substrates W, the chuck, theclamping mechanism used, etc. While the term “average height map” mayrefer to a map based on more than one measurement, a single measurementcould also serve as the basis for an average height map. The informationobtained from a single measurement already contains information aboutthe general shape of a certain type of substrate W. The average heightmap may of course also be based on more than one measurement.

The term “average height map” as used here may be a 2D map comprisingheight information of positions on the surface of the substrate(z(x,y)), but may also comprise rotational information (R_(x)(x,y)). Theaverage height map may also comprise information indicating thatextrapolation of height in the y-direction starting at a certainy-position follows a certain function f(y). So, not always a full 2Drelation needs to the calculated as intermediate step betweenmeasurement of height and applying the extrapolation scheme.

The processing unit 31 constructs an average height map based on themeasurement performed by the sensor 30. The sensor 30 may measure theheight map of the substrate W along a single line AA, perpendicular tothe edge of the substrate W, as depicted in FIG. 5 a, which shows a topview of a substrate W. Based on such a measurement, information may beobtained about the curvature near the edge of the substrate W. Theaverage height map may be assumed equal to this measured height alongline AA, but also processing may be applied to this measured heightalong line AA, such as smoothing out high-frequency variations, as willbe understood by a person skilled in the art. The processing unit maycomprise one or more microprocessors or other programmable units (e.g.digital signal processors) and/or dedicated (e.g. preprogrammed ornonprogrammable) units such as field-programmable gate arrays (FPGAs) orother arrays of logic elements.

However, the sensor 30 may also measure the height map of a substrate Walong more than one line, such as depicted in FIG. 5 a, where the sensor30 measures the height of the substrate W along four lines AA, BB, CC,DD, perpendicular to different portions of the edge of a substrate W.The measured height along these lines may be processed and/or averagedby the processor 31.

Also, the sensor 30 may be used to measure the height of the substrate Win a ring-shaped area along the edge of the substrate W, as shown inFIG. 5 b. A certain type of substrate W could show a different curvaturealong the edge of the substrate, for instance a saddle shape. It will beunderstood that all kinds of measurement strategies may be used. Thesensor 30 may, for instance, measure the height of more than onesubstrate W of a batch and calculate an average height map as averagedover that plurality of substrates W.

Since at least some of these proposed measurements are additionalmeasurements to known exposure procedures, they may cost time andtherefore reduce the throughput. However, it may be feasible to performthem on only a limited amount of substrates W (for instance, one) and/oron a limited area of the substrate W (for instance, a single line AA).

The constructed average height map may be stored in memory unit 32. FIG.6 depicts two examples of possible average height maps I and II. Graph Ishows an average height map with a height increasing towards the edge ofthe substrate W, and graph II shows an average height map with a heightdecreasing towards the edge of the substrate W.

Once the substrates W are measured by the level sensor 15 in order to besubjected to exposure, the information from memory unit 32 is retrieved,for instance, by the same processing unit 31, but it may also be used byanother processing unit (not shown). Once the level sensor cannotmeasure or adequately determine the height or tilt for part of thetarget portion C_(i) that may not be completely on the substrate W, anextrapolation is performed in order to estimate the height map of thatpart of the substrate W. An estimated height map is constructed byextrapolation of height measurements done in neighboring areas.Referring to FIG. 2 b, for constructing an estimated height map of apart of target portion C₁, height measurements may, for instance, beused from target portions C₂, C₃. Also height measurements may beobtained from other parts of target portion C₁. Based on the estimatedheight map, the position and orientation of the substrate W can beadjusted in order to position that part of the substrate W as good aspossible with respect to the focal plane of the optics.

In applications of some embodiments of the invention, extrapolation ofneighboring height measurements can now be done with the use of thepreviously constructed average height map. In case substrate W is of thefirst type, showing an upward average height map I (FIG. 6), theextrapolation could use this information to make a more accurateestimation of the height or tilt near the edge of substrate W. Based onthe obtained average height map, a suitable extrapolation scheme can beconstructed. For instance, if the average height map shows a certainparabolic shape near the edge of the substrate, this same curvature istaken into account in the extrapolation area. Since also the chuck 20influences the curvature of the substrate near the edge of the substrateW, it may be desirable for the construction of the average height map tobe done based on measurements performed on a substrate W positioned onthe same, or a similar chuck 20.

The average height map may or may not provide information about theabsolute height of the substrate near the edge of the substrate. Forexample, it is possible that only the information provided by theaverage height map about the shape of the substrate W near the edge isused. So, the absolute height information may be retrieved from heightmeasurements from neighboring areas on substrate W, while the relativeheight information may be stored in the average height map. The averageheight map may therefore also be denoted by an average profile map.

In the embodiment discussed above, all the different causes that couldinfluence the curvature of the substrate W near the edge of thesubstrate W are taken into account simultaneously. However, it is alsopossible to produce an average height map taking into account the effectof only part of these effects. For instance, an average height map couldbe constructed of the curvature created by the clamping mechanism andthe shape of the chuck 20 and/or the stone 22. Such an average heightmap only needs to be generated once for a certain chuck 22 and/or stone22. Also the influence of the substrate table could be determinedseparately. An average height (profile) map taking into account theeffect of a structure arranged to support a substrate may be determinedby measuring the shape of the structure to support a substrate directly,but may also be determined by measuring a substrate positioned on such astructure (for example, a substrate having a particularly flat surface).

Such an average height map may be used in combination with an averageheight map taking into account the curvature of a certain type ofsubstrate W, or taking into account the effects of processing stepsapplied to the substrate W. So, different average height maps may beconstructed, taking into account different causes. These differentaverage height maps can be used simultaneously. However, it is alsopossible to use an average height map, taking into account only part ofthe possible curvature and use this average height map to improve theextrapolation. Although such a solution may not be optimal, reasonableresults may be obtained.

A method according to an embodiment of the invention includes measuringat least a first part of at least one substrate belonging to a group,wherein the first part corresponds to a portion of the substrate thatmay be measured accurately using the level sensor or other measuringdevice. Computing an average profile map from the measurements taken ofthe at least first part of the substrate belonging to the group andstoring the computed average profile map for use during a laterdetermination of a height or tilt of a substrate from the group. Basedon the average profile map, a profile map may be computed for a secondpart of at least one substrate belonging to the group, wherein thesecond part corresponds to a portion of the substrate that may not bemeasured accurately by the level sensor, or other measuring device, andmay therefore be constructed using extrapolation of the average profilemap from neighboring areas of the substrate. In an alternativeembodiment of the invention, the second part may at least partiallyoverlap with the first part.

A method according to a further embodiment of the invention relates to amethod for determining a first average profile map of at least the firstpart of a substrate belonging to a group of substrates and computing atleast a second part of at least one substrate belonging to the group,based on an extrapolation of the first average profile map. In oneembodiment of the invention, the first average profile map may include apreviously constructed average profile map associated with a first typeof substrate. The method may further include measuring the first part ofa substrate belonging to a group of substrates and storing the measureddata. A second average profile map of the at least first part of thesubstrate belonging to the group may be computed from the measured dataand may be used during a later determination of a height or tilt of asubstrate from the group. Based on at least one of the first averageprofile map and the second average profile map, a profile map may becomputed for a second part of at least one substrate belonging to thegroup. In one embodiment of the invention, the second part may beadjacent to the first part. In an alternative embodiment of theinvention, the second part may be adjacent to the first part and may atleast partially overlap with the first part.

The average profile map may comprise information about the shape of acertain group of substrate. In this text, the term “average height map”may also include an average profile map. This information canadvantageously be used to determine height or tilt in parts of asubstrate of that group that may not be measured by a level sensor. Theterm ‘group’ as used here refers to a group of substrates that aresimilar in some regard; for example, they may have been subjected tosimilar processing steps and/or similar patterned beams, they mayoriginate from the same batch, etc. These substrates are assumed to havea similar shape, for instance, near the edge of the substrate.Extrapolation can be done more accurately with the use of this averageheight map; for instance, the average height map can be used toconstruct a suitable extrapolation scheme. Especially near the edge ofthe substrate, where height measurements are more difficult andrelatively strong curvature of the substrate is often present, such amethod could advantageously be used.

It may be desirable for the first and the second part to at leastpartially overlap with each other.

In a method according to a further embodiment of the invention, thefirst part may include a strip along the edge of the substrate.Especially along the edge of the substrate it may be relativelydifficult to obtain height or tilt measurements, since the level sensormay be partially outside the substrate in that area.

In a method according to a further embodiment of the invention,measurements for the first part may be obtained using at least one line,wherein each of the at least one lines may be perpendicular to the edgeof the substrate.

In a method according to a further embodiment of the invention, thefirst part may be obtained using at least one line, wherein each of theat least one lines may be perpendicular or parallel to the scandirection during exposure of the substrate.

In a method according to a further embodiment of the invention, thefirst part may be obtained using a strip along the edge of thesubstrate. The examples mentioned here for the first part are relativelysimple to apply and provide an average profile map of relatively goodquality.

In a method according to a further embodiment of the invention, thefirst part may be measured using a single spot level sensor. A singlespot level sensor may be capable of measuring a surface with a highaccuracy.

In a method according to a further embodiment of the invention, themeasurement of the first part may be performed while the substrate ispositioned on a same or similar chuck, as on which the substrate ispositioned during the determination of the height. Since the chuck usedto position the substrate may not be completely flat, the specific chuckthat is used may influence the curvature of the substrate. Every chuckmay have a different effect on the curvature of the substrate.Therefore, it may be desirable to construct the average height map basedon measurements taken from a substrate positioned on the same chuck thatis used to support the substrate during exposures. This approach mayonly take into account the influence of the chuck on the shape of thesubstrate. However, the chuck is used many times and its influence mayonly need to be determined once.

In a method according to a further embodiment of the invention, thechuck is provided with a clamping mechanism that is similar to theclamping mechanism used for supporting the substrate during exposures.Also, the clamping mechanism used to hold the substrate in a certainposition may influence the shape of the substrate. Therefore, it may bedesirable to use a similar clamping mechanism during the determinationof the average height map as during exposures, thus making use of levelsensors measurements and the average height map to level the substrate.Such a clamping mechanism could be a vacuum clamping mechanism or aelectromagnetic clamping mechanism. The influence of these clampingmechanism on the shape of the substrate during exposure may only need tobe determined once.

In a method according to a further embodiment of the invention, themeasurements of the first part of the substrate may be performed whilethe substrate is positioned on a same or similar substrate table, as onwhich the substrate is positioned during the exposures. The averageheight map can also be based on information about the shape of thesubstrate table. This approach may only correct part of the causes thatinfluence the shape of the substrate. However, a substrate table is usedmany times, and the influence it has on the shape of the substrate mayonly need to be determined once.

A device manufacturing method according to an embodiment of theinvention includes providing a substrate; providing a projection beam ofradiation using an illumination system; using a patterning structure toimpart the projection beam with a pattern in its cross-section;projecting the patterned beam of radiation onto a target portion of thesubstrate; determining a height or tilt of a portion of the substratewith a sensor, the target portion being adjacent to the first part of atleast one substrate; and determining a height or tilt of the second partof the at least one substrate by extrapolating the height or tilt of thetarget portion of the substrate based on an average profile map obtainedfrom a first part of the at least one substrate as described herein.

In a device manufacturing method according to a further embodiment ofthe invention, the sensor may be a multi-spot sensor.

A lithographic apparatus according to an embodiment of the inventionincludes an illumination system for providing a projection beam ofradiation; a support structure for supporting a patterning structure,the patterning structure serving to impart the projection beam with apattern in its cross-section; a substrate table for holding a substrate;and a projection system for projecting the patterned beam onto a targetportion of the substrate. The lithographic apparatus further is arrangedto determine an average profile map of at least a first part of asubstrate belonging to a group of substrates, and further includes aprocessing unit arranged to communicate with a sensor and a memory unit,the sensor unit being arranged to measure at least the first part of atleast one substrate from that group; the processing unit being arrangedto determine the average profile map of the at least first part of thesubstrate belonging to the group based on the measurement done by thesensor; and the memory unit being arranged to store the average profilemap for use during a later determination of a height or tilt of thesubstrate belonging to the group.

A lithographic apparatus according to a further embodiment of theapparatus includes an illumination system for providing a projectionbeam of radiation; a support structure for supporting patterningstructure, the patterning structure serving to impart the projectionbeam with a pattern in its cross-section; a substrate table for holdinga substrate; and a projection system for projecting the patterned beamonto a target portion of the substrate. The lithographic apparatusfurther is arranged to determine an average profile map of at least afirst part of a substrate belonging to a group of substrates, andfurther includes a processing unit arranged to communicate with a sensorand a memory unit, the sensor unit being arranged to measure at leastthe first part of at least one substrate from that group; the memoryunit being arranged to store the measured data; the processing unitbeing arranged to determine the average profile map of the at leastfirst part of the substrate belonging to said group based on themeasurement done by the sensor during a later determination of a heightor tilt of the substrate belonging to the group.

A method for determining an average profile map according to anembodiment of the invention includes measuring a height profile of atleast part of a structure arranged to support a substrate; computing anaverage profile map of a substrate supported by the structure; andstoring the computed average profile map for use during a laterdetermination of height and tilt values to be used during exposure of asubstrate. Such a support structure influences the height profile of asubstrate that is supported by the support structure. Information aboutthe effect of a certain support structure on the shape of a substratecan be used to construct a more accurate extrapolation scheme fordetermining the height profile of parts of a substrate that cannotaccurately be measured by a level sensor. Such a support structure maynot take into account all the causes that have effect on the shape ofthe substrate, such as the initial shape of a substrate or the effect ofthe various processing steps the substrate has been subject to. However,it may enable a more accurate extrapolation. A further potentialadvantage of such a method is that the average height map of a supportstructure may only need to be determined once for a certain supportstructure and may not have to be repeated for every new batch.

The term “support structure” as used here with respect to a substratemay denote one or more of the following: chuck, clamping mechanism usedto clamp a substrate, substrate table and/or stone. Generally the termalso may denote any structure used to support all or part of a wafer(substrate) or mask (reticle) or other patterning structure. Differentaverage height maps taking into account different parts of the supportstructure may be combined. Average height maps taking into account thesupport structure may also be combined with average height maps takinginto account the shape of a substrate that is not positioned on thesupport structure.

A method for determining an average profile map of at least a first partof a structure arranged to support a substrate according to anembodiment of the invention includes measuring at least the first partof at least one substrate supported by the support structure, the firstpart of the at least one substrate being supported by the first part ofthe support structure; computing an average profile map of the at leastfirst part of the support structure; and storing the computed averageprofile map for use during a later determination of a height or tilt ofa subsequent substrate located on the support structure.

A method for determining an average profile map of at least a first partof a structure arranged to support a substrate according to a furtherembodiment of the invention includes measuring at least a first part ofat least one substrate supported by the support structure, the firstpart of the at least one substrate being supported by the first part ofthe support structure; storing the measured data; and computing anaverage profile map of the at least first part of the support structureduring a later determination of a height or tilt of a subsequentsubstrate located on the support structure. The average profile map mayinclude the effect that the support structure has on the substrate. Suchan average profile may be based on measurements applied directly to thesupport structure, but may also be based on measurements of a substratesupported by such a support structure.

Methods for determining an average profile map of at least a first partof a substrate belonging to a group of substrates are described herein.Device manufacturing methods are also described herein, as well aslithographic apparatus comprising an illumination system for providing aprojection beam of radiation; a support structure for supporting apatterning structure, the patterning structure serving to impart theprojection beam with a pattern in its cross-section; a substrate tablefor holding a substrate; and a projection system for projecting thepatterned beam onto a target portion of the substrate.

Whilst specific embodiments of the invention have been described above,it will be appreciated that the invention as claimed may be practicedotherwise than as described. For example, embodiments of the inventionalso include a memory unit or data storage medium (e.g. a unit or mediumincluding a magnetic or phase-change medium such as a disk or CD-ROM ora volatile or nonvolatile semiconductor medium such as ROM, RAM, SDRAM,or flash RAM) storing instructions (e.g. describing a method asdisclosed herein) that are executable by an array of logic elements(e.g. a processor). It is explicitly noted that the description of theseembodiments is not intended to limit the invention as claimed. The scopeof the invention is determined only by the appended claims.

1. A data storage medium storing instructions executable by an array oflogic elements, the instructions describing a method of determining amap of a surface, said method comprising: measuring a first part of atleast one substrate belonging to a group of substrates; and based on theresult of said measuring, computing a map of a second part of at leastone substrate belonging to the group of substrates, the first part atleast partially overlapping with the second part.
 2. A method ofdetermining a map of a surface of a substrate belonging to a group ofsubstrates, said method comprising: by using a plurality of targetportions, measuring a profile of a part of a surface of a structurearranged to support individual ones of the substrates, said measuringbeing performed while none of the substrates is present on the surfaceof the structure; measuring part of the surface of the substrate; basedon a result of measuring said profile and measuring said part of thesurface of the substrate, computing the map of the surface of thesubstrate supported by the structure; and storing the map.
 3. The methodof determining a map of a surface according to claim 2, whereinmeasuring a profile includes measuring a height profile.
 4. A method ofdetermining a map of a surface, said method comprising: measuring afirst part of a surface of a substrate supported by a structure, using aplurality of target portions; computing a map of at least a part of thesurface supported by the structure, based on the result of saidmeasuring; and storing the map, wherein the map includes an averageprofile map.
 5. The method of determining a map of a surface accordingto claim 4, said method further comprising determining at least one of aheight and a tilt of another substrate based on the map.
 6. A method ofdetermining a map of a surface, said method comprising: measuring afirst part of the surface of a substrate supported by a structure, usinga plurality of target portions; storing a result of said measuring;computing a map of at least a part of the structure, wherein the mapincludes an average profile map; and storing the map in amachine-readable memory or making available the map for use.
 7. Themethod of determining a map of a surface according to claim 6, whereinsaid computing of the map that includes the average profile map isperformed during at least one of a height and a tilt of anothersubstrate.